1. Field of the Invention
The present invention relates to an inspection machine for a printed circuit board, and more particularly to an inspection machine for a printed circuit board including an imaging device having a function (or close-up function) to capture a high resolution image by coming close to the printed circuit board.
2. Description of the Background Art
For example, an inspection machine disclosed in Japanese Patent Application Laid-Open No. H02-52246 has an imaging device in the form of an X-ray irradiation unit and an X-ray camera which face each other with respect to a printed circuit board therebetween.
The X-ray irradiation unit irradiates an X-ray onto the printed circuit board. The X-ray camera receives the X-ray transmitted through the printed circuit board, and captures an image of an inspection target portion of the printed circuit board. Here the resolution of the image captured by the X-ray camera depends on the ratio of a distance from a light source of the X-ray irradiation unit to the printed circuit board (hereafter called “irradiation distance”), and a distance from the printed circuit board to the X-ray camera (hereafter called “transmission distance”). If the so-called close-up function is needed, the X-ray irradiation unit approaches the printed circuit board so that the irradiation distance becomes relatively shorter than the transmission distance.
Recently a close-up function with high resolution is demanded. To meet this demand, it is necessary to move the X-ray irradiation unit close to the printed circuit board so as to minimize the irradiation distance. Therefore, the electronic components mounted on the printed circuit board and the X-ray irradiation unit tends to easily interfere with each other when close-up imaging, that requires high resolution, is performed. A possible method to prevent this interference is measuring the height of each electronic component from a point above the printed circuit board using a distance detection means which measures the distance between the printed circuit board and the X-ray irradiation unit. However, many electronic components are usually dispersed on the printed circuit board. This means that measurement is required for so many measurement points, and therefore measuring the height of each electronic component from a point above the printed circuit board is not practical.
Another possible method is registering data in a server for each printed circuit board to be manufactured, supplying the registered data to an inspection machine and restricting the height to approach each printed circuit board (or each item number). In this case, however, it is possible that the data and actual product may mismatch due to incidents such as data input error, communication error, change of transporting sequence, and due to the state of components mounted irregularly (e.g. part of the components is delicately-mounted on the same point). In other words, data management alone is insufficient solve interference.